Many techniques have been developed for molding plastic lenses. Generally these techniques involve arranging two circular lens molds, known as half molds, in close alignment to form a casting cavity or opening between the two half molds. The half molds are generally made of glass and have inner surfaces which provide the desired curvature on a finished lens. A liquid resin material is inserted in the cavity and cured. Typically, the curing takes place through use of a catalyst by raising the temperature of the resin or by exposing the resin to ultraviolet light and then cooling the resin. One known polymer which has been used is a polyethylene glycol diallylcarbonate with the addition of a catalyst, for example, isopropyl percarbonate, and sold under the trademark CR39. However, the CR39 polymer when exposed to the curing process of exposure to elevated temperatures, has an undesirable shrinkage, sometimes approximately about 14%. The shrinkage causes the resin to pull away from the mold halves which allows air to enter the cavity causing damage to the lens. In certain techniques the half molds are held close alignment with a gasket. In the techniques where light is used to cure the resin, the gasket tends to block the light from curing the resin. In techniques where heat is used to cure the resin, the gasket tends to shrink during cure causing leaking of the resin or deformation of the lens being cast.
An additional drawback is that the CR39 type resins tend to take a long time, typically three hours to seventeen hours to cure. This lengthy curing time adds to the costs and time of preparation of the cast lens.
Another known material is described in the Coleman et al. U.S. Pat. No. 5,708,064 patent assigned to PPG Industries Inc. which relates to a high refractive index photochromic ophthalmic article which comprises various ingredients such as a higher fractive index imparting polymer, a plasticizer and/or a flexible monomer.
Plastic photochromatic lenses have been available in some form since the early 1980's. However, the early designs were of poor quality, and thus, have not been well accepted by the public. One type of photochromatic lens which enjoys the largest market share of photochromatic lens sales involves taking premanufactured lenses and, under pressure, injecting the photochromatic dye into the lens surface. The process allows any lens manufacturer to send lenses to the color processing company and have the lenses converted into a photochromatic lens.
In the past, monomer and dye heat cured lens processes have been unsuccessful. Photochromatic dyes by their nature become unstable when exposed to the high temperatures required to cure lenses. The dyes also tend to react with the organic peroxides used to cure lenses, creating poor quality or non-changing lenses.
In addition, in certain applications, it is desired that the lenses and other substrates, have the capability to change color. Therefore, there is a need for a photochromatic coating material that can be readily applied to a suitable substrate to allow the substrate to change color when exposed to different wavelengths of light.
The present invention is also directed to methods and compositions for coating substrates with photochromatic coating materials which are easier to make and use than prior art coating compositions and which more quickly cure than prior art compositions.
Therefore, there is a need to develop an improved method of molding lenses which does not have these problems.
Therefore, there is also a need in the lens making business for a process to prepare lenses for eye glasses which can be quickly and inexpensively manufactured.
There is a further need for a method for making lenses which can be quickly produced without causing cracking of the plastic lens, optical distortions in the lens, or discoloration of the lens.
Further, there is a further need in the lens making business for a process where the lens and the lens making materials do not undesirably shrink or prematurely separate from the mold prior to curing.
There is also a need in the lens making business for a process to prepare lenses for eye glasses which can be quickly and inexpensively colored or tinted to a customer's desired shade.
The present invention is directed to methods, apparatuses and compositions for making plastic lenses that overcome the above discussed disadvantages and drawbacks in the prior art.